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ARTHROPODA 



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daughter nuclei, which form a blastoderm, at first only on the ventral 

 surface of the egg ; only at a later period do cells come to the surface 

 of the yolk on the dorsal side also. 



When we descend to the lower groups of Crustacea we find that 

 amongst Phyllopoda the development of the Cladoceran genus 

 Polyphemus has recently been worked out by Klihn (1912). In 

 fundamental characters it agrees with that of Eupliausia ; the egg 

 undergoes total segmentation. A 2 -cell stage is followed by a 

 4-cell stage and this by an 8-cell stage in which there are two 

 tiers of four cells, and in which a segmentation cavity or blastocoele 

 makes its appearance. The four cells nearer the animal pole of the 

 egg are larger and clearer than those nearer the vegetative pole, but 

 the latter contain most of the yolk, and in one of them are embedded 

 the remains of the sister cells of the egg, i.e. oocytes, which do not 

 ripen, but serve as nourishment. In the 16-cell stage we get two 



Fig. 142. Stages in the developmeiit of the egg of Polyphemus pediculus. (After Kiihn.) 

 A, passage from 16-cell stage to 30-cell stage, from the side. B, US-cell stage, from below. C, 

 sagittal section through a stage of between 236 and 4o2 cells, end, endodermal cells ; gen, cells of genital 

 rudiment ; mes, mesoderm cells. 



tiers of eight cells each, since every cell except one divides by a 

 meridional cleavage. This exceptional cell is the one of the four 

 situated in the vegetative half of the egg, which has received the 

 remains of the nutritive cells. It divides, not meridional ly, but into 

 an upper and a lower cell; the lower contains the remains of the 

 nutritive cells, it is the rudiment of the genital organs, and is 

 termed the generative cell ; the upper is the endoderm cell, and 

 gives rise to the lining of the mid-gut. At the next period of cleavage 

 these two cells do not divide, but all the other cells divide each into 

 an upper and a lower daughter cell (Fig. 142, A). In this way we 

 get in the animal half of the egg two tiers of eight cells, and in the 

 lower half of the egg an upper tier of six cells and a lower tier of six 

 cells. This lower tier lies at the vegetative pole and forms a horse- 

 shoe-shaped group surrounding the endoderrn cell and the generative 

 cell. There are thus thirty cells in the egg. Shortly after the 

 endoderm cell divides into right and left halves, thus raising the 

 number of cells to 31. 



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